Sharpening high power pulses

ABSTRACT

A metal oxide threshold switch (MOTS, e.g., NbOx, x preferably fractionallyess than 2) series connected is used to sharpen the pulse rise to about one or less nanosecond, with a peak up to several kilovolts, and up to 100 amperes or more.

The invention described herein may be manufactured and used by or forthe Government for governmental purposes without the payment of anyroyalties thereon or therefor.

BACKGROUND OF THE INVENTION

This invention relates to sharpening the rise time of high voltage, highcurrent pulses.

The interest in nanosecond high voltage pulse generations is rapidlyincreasing. At the present time, they are used in quantum physics, innuclear physics, in particle accelerators, in x-ray generators, inhigh-speed photography, etc. The possibility of applying nanosecondgenerators to solve certain problems in experimental physics such as theproduction of powerful pulse lasers, strong-current accelerators ofcharged particles, and fast heating of plasma is of extreme interest.These pulses are also of interest for radar. The state of the art iscovered by a book "Formation of Nanosecond Pulses of High Voltage" by G.A. Mesyats, A. S. Nasibor and V. V. Kremnev, translation available fromNational Technical Information Service, Springfield, VA. 22151, asdocument No. AD 733 130.

A conventional (silicon) solid state device in a pulse-sharpeningcircuit would, because of its 100 to 1000 picofarad capacitance, causeintolerable pre-switching currents. Pulse-sharpening with gaseousdevices is unsatisfactory because of the long and uncertain switch-ondelay (several nanoseconds) with these devices. Therefore pulses fromconventional power sources for lasars and radars are not sharpened--theyhave a much longer than optimal risetime.

SUMMARY OF THE INVENTION

The object of the invention is to provide shorter rise-times with moretime precision (less jitter), while at the same time reducingcomplexity, cost and size of a pulser circuit.

According to the invention, a metal oxide threshold switch (MOTS) isconnected in series between a pulse generator and a load.

One type of MOTS uses NbOx (x equal to or preferably fractionally lessthan 2) as the switching material, either as a thin film formed on NbO,or in bulk form. The threshold voltage may be from about 60 volts up toseveral kilovolts, with a current in the "ON" state of several amperesor more than 100 amperes. The "OFF" state capacitance is only a fewpicofarads, and the switching time less than a nanosecond.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a block and schematic drawing of an embodiment of theinvention; and

FIGS. 2A, 2B and 2C are graphs of voltage and current waveforms.

DETAILED DESCRIPTION

Further information on the operation of metal oxide threshold switchesand some applications is found in two papers by G. Gaule', P. LaPlante,S. Levy and S. Schneider; the first titled Pulse Sharpening withMetal-Oxide Bulk Switching Devices in IEEE International Pulse PowerConference, November 1976; and the second titled Metal Oxide Devices forRapid High Current Switching in the 1976 IEDM Technical Digest, pages279-282, published December 1976. These papers are made a part hereof.They include a disclosure of the invention claimed herein.

Metal oxide threshold switches, MOTS, have been used to suppresstransient pulses such as induced by lightening, etc. See, for example,U.S. Pat. No. 3,973,224. When applied in that manner, the device isplaced in shunt across a transmission line to absorb the transient pulsecurrent, and thereby protect a load device at the end of the line.

When a metal oxide threshold switch is subjected to a fast rising pulsefrom a low impedance source, it goes from a high impedance state(characterized by typically 100 kilohms to one megaohm in parallel with1 to 4 picofarads) to a low impedance within less than a nanosecond. Thevoltage then appears clamped to the threshold value, and subsequentlydrops to a "holding voltage" of typically 20 volts. After termination ofthe applied pulse, the high impedance state, and the previous thresholdvoltage value is essentially recovered within typically 6 microseconds.

One form of the metal-oxide (MOX) chip is a NbO (a semiconductor, actingas ohmic contact) crystal with a 10 micrometer NbOx layer thermallygrown on top by oxidation. (See U.S. Pat. No. 3,962,715.) Thestoichiometric value of x is 2, but values lower than that, for example,x equal to about 1.9 have been found preferable for switching material.The thin film form yields threshold voltages from 80 to 600 volts. Forhigher voltages, single crystal chips of NbOx are used. Thresholdvoltages are controlled by the thickness of di-oxide and also thedeviation from stoichiometry. The current flow in the low impedancestate is typically several amperes, and values of 250 amperes have beensustained.

Referring now to FIG. 1, the basic circuit comprises a pulse generator10 supplying pulses to a load represented by a 100-ohm resistor 24. Tosharpen the pulses, a metal oxide threshold switch device 20, forexample a NbOx device, is connected in series between the source and theload.

In an experimental arrangement, a Cober pulse generator 10, type 605-P,was set to produce a slow rising pulse (100 nanoseconds). The reversingswitch 12, transformer 14, and 1500-picofarad capacitor 16 may beincluded as a part of the pulse source. To observe the results anoscilloscope, type Tektronix 7904 and a 10-ohm resistor 22 are connectedas shown.

The applied voltage from the pulse source is shown in FIG. 2A. Theeffectiveness of pulse sharpening is tested by replacing the MOTS device20 with a short. In that case, the shape of the current pulse resemblesthat of the applied voltage (FIG. 2B). Insertion of the MOTS device 20causes sharpening of the current pulse, as shown in FIG. 2C. The currentrise time is thus reduced to a few nanoseconds. During repetitivepulsing there is some variation in the time of the onset of the sharprise of the current (jitter). However this "time jitter" can be quitesmall compared to that obtained with prior high voltage, high currentswitching devices.

What is claimed is:
 1. Pulse sharpening means for a circuit in which apulse source supplies high voltage, high current pulses to a load; thepulse sharpening means comprising a metal oxide threshold switch deviceconnected in series between the pulse source and the load, to therebylimit the current flow to the load to a very small leakage value until athreshold voltage value is reached to switch said device from a highimpedance to a low impedance state, with a fast rise time of the currentpulse to a high value, the voltage across said device dropping to a lowholding value; and said device returning to the high impedance stateresponsive to the voltage across it becoming less than the holding valueas the pulse from the pulse source terminates.
 2. Pulse sharpening meansaccording to claim 1, wherein the metal oxide of said device is NbOx,the value of x being in the range from approximately 2 extendingdownward into non-stoichiometric values.
 3. Pulse sharpening meansaccording to claim 2, wherein the threshold voltage is in the range fromapproximately 60 volts to several kilovolts, and the current flow in thelow impedance state is in the range from several amperes to hundreds ofamperes.